Non-alcoholic fatty liver disease (NAFLD) is the most common cause of abnormal liver function tests in the United States, and it can progress to cirrhosis, hepatocellular carcinoma, need-for- liver transplantation and death. Fatty liver disorders are polygenic diseases that are associated with obesity and the metabolic syndrome, however, the pathogenesis remains poorly understood. Human studies demonstrate that the Unfolded Protein Response (UPR) is important in the pathogenesis of fatty liver; and dysregulation of the XBP1s pathway of the UPR is associated with the progressive form of NAFLD termed non-alcoholic steatohepatitis (NASH). The overall objectives of this grant proposal are to further determine the role of Xbp1s in the pathogenesis of non-alcoholic fatty liver disease and identify novel regulatory factors and pharmacologic agents that can provide therapies for this common disease. We have recently developed mice with a hepatocyte-specific deletion of Xbp1s and will utilize these mice to determine the role and mechanisms of Xbp1s signaling on hepatic: A) injury in vivo using mice fed High-Fat diets; and lipotoxicity in vitro usng Huh7 cells and primary hepatocytes (Specific Aim 1). We will subsequently employ Quantitative Trait Loci (QTL) analysis and positional cloning of mice fed a High-Fat diet to identify novel genes and gene modifiers that are important in the pathogenesis of fatty liver diseases (Specific Aim 2). Finally, we will perform a high-throughput chemical screen to identify compounds that enhance hepatic XBP1s expression using Huh-7 cells with a XBP1- splicing luciferase reporter gene construct (Specific Aim 3). This proposal combines molecular biology, forward and reverse genetics, and high-throughput approaches to further determine the pathogenesis of steatohepatitis and develop therapies for non-alcoholic fatty liver diseases.